1. Field of the Invention
The present invention relates to a SAW (Surface Acoustic Wave) filter and more particularly to a SAW filter having an improved attenuation characteristic at a frequency that is any multiple of an attenuation pole frequency at one or both sides of a pass band.
2. Description of the Background Art
Conventional technologies relating to a SAW filter are disclosed in, e.g. Yoshio Satoh, et al, “A Low-Loss Band-Pass Filter Using SAW Resonators”, Transactions of the Institute of Electronics, Information and Communication Engineers of Japan, Vol. J76-A, No. 2, pp. 245-252 (1993), Mitsutaka Hikita, et al, “Experiments of High-Performance SAW Filters for Mobile Radio Transceivers”, Transactions of the Institute of Electronics, Information and Communication Engineers of Japan, Vol. J76-A, No. 2, pp. 236-244 (1993), and Yuji Suzuki, et al, “Coupled-Mode Theory of SAW Periodic Structures”, Transactions of the Institute of Electronics, Information and Communication Engineers of Japan, Vol. J76-A, No. 2, pp. 87-95 (1993). SAW filters taught in this document each include one-terminal pair SAW resonators connected in a ladder configuration. The one-terminal pair SAW resonators constitute serial arms and parallel arms forming a high-frequency attenuation pole and a low-frequency attenuation pole, respectively.
FIG. 1 shows a specific conventional constant K SAW filter in which SAW filters are cascaded in four consecutive stages. This four-stage configuration is also applied to the present invention by way of example. More specifically, as shown in FIG. 1, the constant K SAW filter has a first stage made up of a serial arm resonator 401 and a parallel arm resonator 501, a second stage made up of a serial arm resonator 402 and a parallel arm resonator 502, a third stage made up of a serial arm resonator 403 and a parallel arm resonator 503, and a fourth stage made up of a serial arm resonator 404 and a parallel arm resonator 504. The first to fourth stages are cascaded, as illustrated.
The ladder type SAW filter shown in FIG. 1 has the following problems when a small loss and a great attenuation are desired. First, the SAW filter cannot achieve a small loss or a great attenuation unless the quality factor of resonance and that of antiresonance of the one-terminal pair SAW resonators are increased. Stated another way, it is necessary to reduce the attenuation in the pass band and to increase it at the attenuation pole.
However, if the transposition length or the pair number of comb line electrodes or interdigital transducers (IDTs) is increased in order to increase the quality factor of resonance of the one-terminal pair SAW resonators, then the damping capacitance of the resonators increases while the antiresonance quality factor of the resonators decreases, reducing the attenuation. It is to be noted that damping capacitance refers to capacitance that the resonators have even at a frequency extremely close to DC, i.e. when they do not oscillate. On the other hand, to implement the attenuation value, it is necessary to reduce the transposition length or the pair number of the interdigital transducers. This, however, lowers the quality factor of resonance and thereby aggravates an insertion loss.
Second, as for a mobile telephone extensively used today, it is necessary to attenuate the secondary and tertiary distortion of a power amplifier included in a transmitter circuit. Therefore, the prerequisite with a filter for the duplexer is that it increases an attenuation at two- or threefold harmonics, i.e. harmonics two or three times as high as a resonance or an antiresonance frequency. The one-terminal pair SAW resonators, however, each simply play the role of capacitance at a frequency remote from the resonance frequency. Consequently, as shown in FIG. 2, it is difficult to implement a desired attenuation over a broad range up to the harmonics mentioned above.
If additional L and C circuit components are connected to the filter in order to attain the desired attenuation, then the original characteristics in the pass band and attenuation band are also deteriorated because the resonance quality factor of the LC circuit is low.
As stated above, it is difficult to reduce the insertion loss in the pass band while maintaining a great attenuation up to the two- or threefold harmonics of the resonance or antiresonance frequency.